Lately, I have received numerous questions about grease. The general theme of most of them is “why can’t I use synthetic grease from the local parts store in my aircraft?”
Synthetic lubricants are one of the present “now” or “buzz” words. If a lubricant is synthetic, people believe it will last forever and lubricate anything.
Synthetic-based oils do offer some advantages, especially in extreme high or low temperatures. But they are not the end-all, cure-all that people are hoping for.
If we go back to the basics of grease, grease is not thick oil. It is normal oil base stocks that are chemically thickened.
The thickening agent varies depending on usage and base oil. This is critical to this question because not all thickening agents are compatible.
If you use a grease that is not compatible with the grease already in the bearing, you can end up with a soup-like substance that will leak out of the seals, leaving the bearings unprotected.
One reader who wrote in said he planned to clean the bearings and start with all new grease. This limits the compatibility problem now, but what will happen down the road if he sells the aircraft? Will the new owner try to revert back to the proper grease and have a failure?
This is why pilots need to do their homework when buying a used aircraft. Always check the maintenance records to ensure that the previous owner lubricated the aircraft properly at the correct intervals, using the recommended spec grease. If not, you may need to completely clean the bearings and start over with the correct product.
A second major problem with non-aviation lubricants is seal compatibility. Many synthetic greases will shrink seals meant for mineral oils only. This shrinkage can allow grease to leak out and water and dirt to enter the bearing area and significantly reduce the life of the bearing.
A third major problem — especially with synthetic greases — is centrifugal force.
Many years ago, Hartzell Propellers wanted to change the spec for the grease used in its props. Historically the company had used a grease-like Aeroshell grease 5 in the props, but this product is based on a thick mineral oil and was a little stiff for some turboprop applications, especially at high altitudes.
The company tried several synthetic products, but all seemed to have a problem. That’s because synthetic base oils are great lubricants, but very poor solvents. This resulted in the thickeners being separated from the base stocks under high centrifugal forces. The thickeners plugged up the prop and the thin base oils leaked all over the outside of the prop.
The bottom line is in an aircraft there are technical reasons why you need to stick to only those products that are qualified for your application.
I know that many general purpose greases seem to meet the same specs as the mil spec for qualified products for your aircraft. But there are a lot of differences that can make a significant difference in the performance and safety of your aircraft.
Personally I have always used synthetic grease, the same kind I use on my buddy boat bearing on the trailer, I have owned several airplanes with absolutely no bearing problems of any kind. I also think the annual time of removing the wheel brgs. on an airplane and repacking them is a waste of time, here’s why I say that. During the course of one year you taxi and take off and land your aircraft around 150 miles on the average, Yet you drive your vehicle anywhere from 20 to 30 thousand miles a year without removing the wheel bearings and repacking them, They use the same cup and cone type of timken bearing that are used on aircraft !! So why the overkill?? I use boat bearing grease because it’s water prof and never lets me down, where as the aircraft specialty types of grease have let me down. As for the prop w/zerks I only pump in 2 or 3 pumps of a hand grease gun then I cycle the prop, In almost 40 years of flying and maintaining my own aircraft I have never had a failure of any kind. I strongly believe in using common sense in a lot of the alleged mandatory maintenance projected in aircraft manuals. I remember when most greases base was comprised of clay as a base product to get the firmness of the grease. It also seems to make sense, to continue to lube as needed more so than waiting for the “proper” grease or specialty type of grease that for some reasons they don’t always work. Just my opinion, nothing personnel, thanks.
I assume you also make up your own specs. On the aircraft you fly. I would hate to be the person that’s buys your airplane. I bet you also do work on your airplane that a A&P should be doing.
What the heck do you think the manufacturer puts out certain numbers and specs. You sir are a danger to yourself and others around you.
This is a great reason why owner assisted annuals are a good idea. The consistent oversight during annuals done by different mechs and IA’s eliminates this problem of mixing greases. The really important part to lube with the proper grease is the prop. My McCauley has no zerks. One less thing to worry about.
The same principle applies to car maintenance, although the newer cars are essentially zerk fitting free. I wonder how much premature ball joint failure can be attributed to different mechs using different greases over time?
Education and maintenance self-involvement proactively prevents problems.
Just because an oil or a grease is marketed as “synthetic” doesn’t mean it’s so much better. With regular maintenance (oil changes, greasing) , using what the airframer, engine or prop manufacturer specifies, then you are doing the best you can.
As for used aircraft someone may purchase, I have found that most maintenance records may not reflect what oil or grease has been used.
In our Cessna dealership, we used nothing but Aeroshell Oil “W”, and Aeroshell grease 4 and 5. Never had an issue.
Oh. And as for synthetic AvOil, you can get it, but don’t use it with leaded AvGas. Your engine will clog up with lead sludge.
Ben,
Thanks for the info and cautions on synthetic grease.
I had no idea how a prop would be affected.
Our 1961 Cessna 175B only gets what the manual specifies…..except for using Camguard in the engine oil.
[ can I mention that ?]